Hypothermia occurs commonly during surgery, and is associated with an increased incidence of myocardial ischemia in the early postoperative period. The mechanisms for cold induced myocardial ischemia are unclear, but two primary thermoregulatory responses (vasoconstriction and shivering) may be implicated. Vasoconstriction is adrenergically mediated and acts to conserve heat, while shivering produces heat by increasing metabolic rate. These adrenergic and metabolic responses could adversely affect the cardiovascular system, and it is therefore important to understand what triggers these physiologic changes, and how they are modulated. In the proposed study, the primary specific aim is to determine how skin-surface temperature modulates the response to core hypothermia. We will assess the adrenergic response by measuring plasma catecholaniines, vasoconstriction, and hemodymic changes. The metabolic response will be assessed by shivering and total body oxygen consumption. The determination of age-related differences in the response to cold stress will provide insight into the mechanisms of cold-induced myocardial ischemia. Determination of the relative contribution of core and skin-surface temperatures to the thermoregulatory response will provide an understanding of how cold- induced physiologic changes can be modulated in patients who develop perioperative hypothermia.